How to make a simple plane slightly transparent?
Just to be on the safe side: Create a new material, assign it to one of your planes. in the  Shader editor you get: 


Adjust the alpha to your likings. 
You should be able to see the same options in the material properties, when you clicked use nodes. You need to be in rendered view to preview any transparency effects. Make sure to check 'use nodes'.
[Big EDIT]
If you want to see the effect in preview: 
This way you can't set the transparency per object, just for all.
You will soon be able to use these settings for individual objects in solid view: